Behavioral studies have shown similarity of action between benzodiazepines (BDZs) and facilitation of GABAergic transmission, indicating that the pharmacological action of BDZs is mediated through GABAergic synapses. The molecular composition of the GABA receptors was studied biochemically. Two separate binding sites (one for 3H-GABA and one for 3H-diazepam) were isolated by differential solubilization from rat brain homogenates with Triton X-100. Another element of the GABA receptor complex has been isolated, an endogenous brain peptide (DBI) that probably represents the endogenous ligand for the BDZ receptor. DBI was prepared from rat brain by extraction in hot 1N acetic acid and was purified to homogeneity by Sephadex and ion exchange chromatography and by reverse phase HPLC. Purified DBI contains 10 to the 4 amino acid residues and is basic in nature. This material inhibits competitively 3H-diazepam and 3H-beta-carboline. Behavioral and binding studies indicates that DBI is an effector for the benzodiazepine receptor with similarities to the anxiogenic Beta-carbolines. These data suggest that GABA receptors and BDZ receptors are functionally associated and that an endogenous peptide regulates their function.